My impression is that if someone paid for placement, Amazon will label the items as such. So in a detail page you'll see "Frequently bought together" (not paid

My impression is that if someone paid for placement, Amazon will label the items as such. So in a detail page you'll see "Frequently bought together" (not paid placement), "Sponsored products related to this item" (paid), "Customers who bought this item also bought" (not paid). Whether or not you think it's an ad is up to you... they're all trying to get you to buy more. :-)